The entry A128729 in [1]} is a bit mysterious. It presents some results about skew Dyck paths without a (contiguous) sequence
`up–down–left,' but without justification, and without reference to a research paper. The present note is designed to fill these gaps.
First, skew Dyck paths are Dyck paths with additional left steps \((-1,-1)\)  but without overlapping itself. We found it more convenient [2]}
to replace a left step by a red down-step \((1,-1)\) , labelled red. So we might say that up–down–red is forbidden. In a second step, it won't be forbidden, but counted how often this happens. If it does not happen, it is the same as `forbidden'. Probably the first paper that deals with skew Dyck paths is [3]}.
